Unsupported Michelson features

No validity checks of string representations of crypto-constants

Crypto-constants, addresses, keys, key hashes and signatures are expressed in SCaml as strings wrapped by variant constructors, Address "tz1gjaF81ZRRvdzjobyfVNsAeSC6PScjfQwN" for example.
These strings must have proper formats for each crypto types but they are not checked by SCaml. They must be checked by Michelson type-checker.